Method and apparatus for reducing visibility of damping wires in aperture grill display tubes

ABSTRACT

A method and apparatus for reducing visibility of damping wire artifacts in aperture grill display tubes comprises a sensor device for locating the artifacts and responsively generating amplitude values, a processor for receiving the generated amplitude values and responsively calculating correction values, and a compensator device coupled to the processor for utilizing the correction values to correct said artifacts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to video display monitors and moreparticularly to a method and apparatus for reducing visibility ofdamping wires in aperture grill display tubes.

2. Description of the Background Art

Accurate representation of visual information is a significantconsideration of manufacturers, designers and user of video displaymonitors. Aperture grill cathode ray tubes (CRTs) are devices which areoften used in conventional video display monitors.

Referring now to FIG. 1, a diagram of a display CRT 110 is shown,according to the present invention. CRT 110 includes a glass screen 112with an inner surface that is covered with a phosphor coating 114. Inaperture grill CRTs, an electron gun 116 causes electron beams 118 topass through vertical slits etched in a thin sheet of metal whichcomprises an aperture grill 120. The slits are part of the mechanismthat directs the red, green and blue electron beams 118 to theirrespective phosphors 114. The electron beams 118 emitted from theelectron gun 116 assembly pass through the aperture grill 120 on the wayto the phosphor coating 114, which responds to the electron bombardmentby emitting light. The aperture grill 120 partially blocks the electronbeams 118, casting shadows on the phosphor coating 114. The shadows lineup with black stripes that separate the red, green and blue verticalphosphor stripes on phosphor coating 114, and because of the fine pitchand regular spacing, the vertical stripes are usually not noticed.

Referring now to FIG. 2, a diagram of the FIG. 1 aperture grill 120 isshown, according to the present invention. Because the metal aperturegrill 120 is stretched taut and welded in place over a metal frame, thesheet has a tendency to “ring” in response to mechanical stimulus, suchas mechanical shock or coupled acoustic power. Such ringing causesdistracting time-varying luminance modulation as the shadows of theaperture grill 120 beat with the pattern of the phosphor stripes onphosphor coating 114.

A countermeasure employed to minimize the extent of the ringing is theaddition of one or more damping wires 210 stretched across the grill 120assembly, perpendicular to the vertical slits. As a result of frictionwith the moving aperture grill 120 and internal elastic losses, thiswire dissipates mechanical energy to thereby reduce the extent of theringing. The damping wires 210 cast a shadow on the phosphor-coating114, but since the damping wire 120 diameter is smaller than that of thevisible beams 118 cross section, the shadow is of relatively lowcontrast. There are typically two damping wires 210 present if CRT 110is 17 inches or larger and there is typically one damping wire if CRT110 is smaller than 17 inches. The damping wire shadow is caused by themodulation of the amplitude of the electron beams 118 incident on thephosphor coating 114.

Since there are only one or two damping wire 210 shadows on the screenof CRT 110, and they are perpendicular to the regular fine-pitchstructure of the vertical stripes, they are sometimes easily observed,though often not distracting. None the less, when first observed, theseshadows are often perceived by as defects in CRT 110. This perception issometimes a factor in customer satisfaction and overall perception ofimage and product quality. Therefore, an improved method and apparatusfor reducing visibility of damping wires in aperture grill display tubesis needed.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and apparatus aredisclosed for reducing visibility of damping wires in aperture grilldisplay tubes. This invention reduces the visibility of damping wireshadows by modulating the amplitude of the CRT electron beam tocompensate for the modulation imposed by the damping wires, thuscanceling the damping wire shadow. The invention is composed of anapparatus and technique to locate the damping wire shadow with respectto the displayed video image, and an apparatus that generates thecompensation signal and applies to the video signal to compensate forthe damping wire shadow.

In the preferred embodiment of the present invention, a detector devicemeasures amplitude values of selected areas on a CRT screen asindividual CRT scan rows are sequentially illuminated. The measuredamplitude values are then processed with a differentiation routine todetermine the specific location of the damping wire shadow on the CRTscreen.

Correction values to compensate for the damping wire shadow arecalculated by subtracting the scan row amplitude value at the dampingwire shadow from an average of the scan row amplitude values for the twoscan rows adjacent the damping wire shadow. The correction values arethen converted into corresponding video values which are storedsequentially and alternately into two display lists along withcorresponding location information such as column and row addresses.

A compensator device then sequentially provides the video values andtheir corresponding location information to video amplifiers to drivethe display CRT. The present invention thus advantageously compensatesthe CRT electron beam current at the appropriate time to effectivelyremove the damping wire shadows from the aperture grill display tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a display CRT according to the present invention;

FIG. 2 is a diagram of the aperture grill from the FIG. 1 display CRTaccording to the present invention;

FIG. 3 is a block diagram of a computer system including the display CRTof FIG. 1 and a damping wire compensator, according to the presentinvention;

FIG. 4 is a flowchart of a method for reducing visibility of dampingwires in a display CRT, according to the present invention;

FIG. 5 is a block diagram illustrating a general method for determiningdamping wire locations in a display CRT, according to the presentinvention;

FIG. 6 is a block diagram illustrating a preferred method for measuringscan row amplitudes in a display CRT, according to the presentinvention;

FIG. 7 is an graph of sample scan row amplitudes used to determinedamping wire locations according to the present invention;

FIG. 8 is a graph of the FIG. 7 sample scan row amplitudes after theyare processed using a differentiation routine, according to the presentinvention;

FIG. 9 is a graph of sample scan row amplitudes used to calculate acorrection value according to the present invention;

FIG. 10 is a block diagram of display lists according to the presentinvention; and

FIG. 11 is a schematic diagram of the preferred embodiment for thedamping wire compensator of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses a method and apparatus for reducingvisibility of damping wires in aperture grill display tubes andcomprises a sensor device for locating the artifacts and responsivelygenerating amplitude values, a processor for receiving the generatedamplitude values and responsively calculating correction values, and acompensator device coupled to the processor for utilizing the correctionvalues to correct said artifacts.

Referring now to FIG. 3, a block diagram of a computer system 310 isshown, according to the present invention. Computer system 310preferably comprises a central processing unit (CPU) 312, a display CRT110, a keyboard 316, an input device 318, a damping wire compensator 322and a memory 324. Memory 324 typically contains an operating system andat least one application program (not shown), and a compensation engine328. In the preferred embodiment, the compensation engine 328 is asoftware routine that provides a set of instructions to CPU 312 forreducing visibility of damping wires 210 in CRT 110. The operation ofcompensation engine 328 is further described below in conjunction withFIG. 4. Each element of computer system 310 preferably has an input andan output coupled to a common system bus 326. Memory 324 mayalternatively comprise various storage-device configurations, includingRandom-Access-Memory (RAM), Read-Only-Memory (ROM), and non-volatilestorage devices such as floppy-disks and hard disk-drives. System bus326 may alternatively be connected to a communications interface topermit computer system 310 to output information to a computer network.

Referring now to FIG. 4, a flowchart of a method for reducing visibilityof damping wires 210 is shown. In the preferred embodiment, the presentinvention causes CPU 312 to coordinate and perform the method steps ofFIG. 4 in response to the program instructions of compensation engine328. In alternate embodiments, the FIG. 4 processes may be performedusing hardware implementations or firmware devices. In step 410, thepresent invention locates the positions of all damping wires 210 presentin the aperture grill 120 of CRT 110. Techniques for locating dampingwires 210 are further discussed below in conjunction with FIGS. 5-8.

Then, in step 412, the present invention calculates correction valueswhich correspond to the location and intensity of the damping wires 210.Further details for calculating correction values are discussed below inconjunction with FIG. 9. Next, the present invention, in step 414,builds display lists 320 by converting the calculated correction valuesinto corresponding video values and then storing the converted videovalues along with their corresponding locations on the CRT 110 screen.Construction of display lists 320 is further discussed below inconjunction with FIG. 10. Finally, in step 416, the present inventionapplies the converted video values to a video amplifier in CRT 110 usingdamping wire compensator 322 which is further discussed below inconjunction with FIG. 11.

Referring now to FIG. 5, a block diagram illustrating a general methodfor determining damping wire 210 locations is shown. FIG. 5 depicts afrontal view of display screen 112 and the location of damping wires210. Also depicted is a measurement area 510. In practice, severalmeasurement areas 510 are imaged (possible while performing whiteuniformity compensation) preferably using a conventional light-sensingdevice. Some of these measurement areas 510 will contain images of thedamping wire 210 shadow (such as the six areas indicated in FIG. 5).

The location of each of the measurement areas 510 relative to theirscreen 112 location is known, and they are analyzed to determine theposition of the damping wire 210 shadow (all the way across the screen112) and the transmission reduction due to the shadow at eachmeasurement area 510 affected by the shadow. The amount of attenuationof the shadow at each measurement area 510 can be measured to determinethe amount of compensation required, or a nominal correction for theparticular display CRT 110 under consideration can be used. These dataare then used to generate a display list 320 (FIG. 3) for the dampingwire compensator 322. Alternately, the location of the damping wire 210shadow may be found by using a spatial imaging device. For example, atelevision camera may be used to image an area of the raster to locatethe damping wire 210.

Referring now to FIG. 6, a block diagram illustrating the preferredmethod for determining damping wire 210 locations is shown. Initially,screen 112 of CRT 110 is not illuminated by electron beams 118. Themeasurement area 510 in screen 112 is then imaged by measuring screen112 luminance as segments of pixels are illuminated, one horizontal scanrow 610 at a time, sequentially advancing the horizontal scan row 610from top to bottom of measurement area 510, where the user has placed aphoto detector. The relative luminance resulting from each measured scanrow 610 is stored as an amplitude value and the amplitude values arethen used to determine the location of the damping wire 210.

Referring now to FIG. 7, a graph of sample scan row 610 amplitudes isshown. The specific FIG. 7 amplitude values are presented for purposesof explanation and may readily include other measured amplitude valuesin alternate examples. The FIG. 7 graph displays successive scan row 610samples on the horizontal axis and corresponding measured scan row 610amplitudes on the vertical axis. In the FIG. 7 example, the damping wire210 shadow corresponds with sample scan row number 3 because scan rownumber 3 has the lowest measured amplitude. In other embodiments,damping wire shadow 210 may be located on another scan row 610, andfurthermore, measurement area 510 may readily comprise a different totalnumber of scan rows 610.

Referring now to FIG. 8, a graph of FIG. 7 sample scan row 610amplitudes is shown after they have been processed using adifferentiation routine, according to the present invention. Thedifferentiation routine may be performed by subtracting the scan row 610amplitude of each sample from the scan row 610 amplitude of the previoussample. The differentiation values thus are equal to the differencebetween successive adjacent scan rows 610. After differentiation, thereverse in slope between the second and third scan row 610 samples andagain between the third and fourth scan row 610 samples indicates thatthe location of the damping wire 210 shadow corresponds with samplenumber 3.

The differentiation routine removes the low-frequency variation inamplitude that results from non-uniform sensitivity of the photodetector probe and facilitates location of damping wire 210. Theamplitude of the particular scan row 610 of damping wire 210 may then becompared with the two adjacent scan rows 610 to determine the amount ofcorrection needed to compensate for the damping wire 210 shadow.

The error caused by the damping wire 210 shadow may be found bysubtracting the FIG. 8 differentiation value at the location of thedamping wire 210 shadow (here, sample scan row number 3) from theaverage of the two adjacent differentiated scan rows 610 (here, scan rownumbers 2 and 4). This error value may then be compared to the amplitudeof the undifferentiated amplitude value of scan row 610 at the dampingwire 210 location to determine the amount of correction required tocompensate for the damping wire 210 shadow.

Referring now to FIG. 9, a graph of sample scan row 610 amplitudes usedto calculate a correction value is shown, according to the presentinvention. FIG. 9 shows a scan row 610 amplitude value A which is theaverage of the amplitudes for the scan rows (here, scan rows 2 and 4)which are adjacent to the scan row 610 that is shadowed by damping wire210 (here, scan row 3). FIG. 9 also shows a scan row 610 amplitude valueB which is the amplitude of the scan row 610 at the location of thedamping wire 210.

In the preferred embodiment, the intensity of the electron beam 118 atscan row 3 must be increased by a factor equal to A/B to compensate forthe shadow cast by damping wire 210. In other words, the amplitude Bmust be increased by a value equal to A minus B, to effectively raisethe amplitude of value B until it equals the amplitude of value A.Alternatively, once the location of the damping wire 210 shadow isfound, successive correction values may be applied to that locationuntil the amplitude at that location equals the average of the twoadjacent locations.

Referring now to FIG. 10, a block diagram of sample display lists 320 isshown, according to the present invention. The FIG. 10 display lists 320are presented as an example for purposes of discussion, and in alternateexamples, other quantities of video values may be stored in displaylists 320. The present invention requires the display system to be ableto relate data signal levels (correction values) to video drive levels.The information required to do so may be obtained by monitoring the beamcurrent 118 while presenting various correction values to the videoamplifier and inferring an intensity from prerecorded luminoussensitivity data. U.S. Pat. No. 5,512,961 discloses and teaches relatedtechniques and is therefore hereby incorporated by reference.

In the preferred embodiment, CPU 312 converts the calculated correctionvalues into corresponding video values and then stores the video valuesinto display lists 320. The conversion process is typically performedthrough the use of a conversion table which reflects the luminancecorrection transfer function of CRT 110. The conversion table may beconstructed by measuring display screen 112 for those luminance changeswhich correspond to the range of possible correction values. CPU 312 maythen readily convert calculated correction values into video values byreferencing the compiled conversion table.

In practice, display lists 320 include display list A 1010 and displaylist B 1040 which each comprise a series of video values andcorresponding row and column addresses. Each video value represents thecalculated amount of adjustment needed to compensate for the dampingwire 210 shadow at a given location on screen 112. The row and columnaddresses correspond to the specific location on screen 112 to which aparticular video value pertains.

The display lists 320 are sets of addresses and video values that areapplied whenever the corresponding addresses are encountered by beam 118on screen 112. As such, a nominal value relating to no gain increase isapplied at the start of the first row and column, and then when anotherrow and column (whose address corresponds to an entry in the displaylists 320) are encountered, the corresponding video value for thataddress is applied to the beam current 118. The display lists 320 arekept in the same sequence as the addresses are generated, and alternateentries are kept in two sets of display list A and B (1010 and 1040) sothat the data from one display list can be read out while the contentsof the other display list is being fetched. This approach allows the useof lower speed processes.

The scanning of screen 112 (scan row 610 by scan row 610) to detect thelocation of the damping wire 210 shadow (if a spatial imaging device isnot used) is ideally performed by loading the appropriate address andvideo values into the display lists 320 and then using them to drive theCRT 110 video amplifier inputs rather than the gain modulation inputs.Alternatively, the host system 310 (the system providing the videosource) can provide a signal corresponding to a full white raster, andan extra bit or reserved value in the video value register can be usedto blank screen 112 in all but the selected row segments.

Referring now to FIG. 11, a schematic diagram of the preferredembodiment for the damping wire compensator 322 is shown. The presentinvention's correction signals are preferably generated by a list-drivendamping wire compensator 322 as shown in FIG. 11. Timing and controlsignals derived from the horizontal and vertical video synchronizationpulses are used to address the display list A 1010 and display list B1040 to select which display list 320 correction value is fed to D-to-Aconverter 1190 to drive the gain modulation input of the videoamplifiers(not shown) in CRT 110.

In the preferred embodiment, control logic 1110 provides an addresscorresponding to the current location of electron beam 118 (on screen112) to address comparator 1170. Display list A 1010 and display list B1040 contain alternate sequential entries of video values and theircorresponding column and row addresses, as described above inconjunction with FIG. 10. Initially, control logic 1110 gates row/columnaddress information from display list A 1010 into multiplexer (MUX)1140, and simultaneously provides data (video values) from display listA 1010 into multiplexer (MUX) 1160.

Control logic 1110 preferably selects the “Display list A” input of MUX1140 to provide the row/column address to address comparator 1170 andthe corresponding correction value to data latch 1180. When therow/column address of the correction data matches the current address(location) of the electron beam 118, address comparator 1170 gates thecorrection value from data latch 1180 through D-to-A converter and on tothe gain modulation circuit of the video amplifiers controlling electronbeam gun 116.

Address comparator 1170 then sends an acknowledge signal to controllogic 1110 to report that the correction value has been sent to the gainmodulation circuit. Control logic 1110 then responsively repeats theabove process using display list B 1040 as the source of the correctioninformation. By repeating the above sequences, compensator 322 thusalternately and sequentially fetches correction information from the twodisplay lists (1010 and 1040) and thus effectively compensates for theshadow caused on display screen 112 by damping wire 210.

A number of additional considerations and alternate techniques should bementioned in connection with the present invention. For example, thecorrection signal may be applied to the gain modulation inputs of theRed, Green and Blue video amplifiers. It should only be necessary togenerate one signal to apply to all three channels, but in the case of asystem with severely degraded vertical convergence, separate correctionsfor Red, Green and Blue may be worthwhile.

For the case in which two or more scan rows 610 are affected by a singledamping wire 210 shadow, the amplitudes of the correction signals on thetwo damping wires 210 can be adjusted so they both contribute to thecompensation in proportion to their respective affects from the dampingwire 210 shadow. In other words, each line would be compensated justenough to bring its amplitude over that segment up to the level it wouldbe at if there were no shadow.

The FIG. 11 data latch 1180 is only needed to de-glitch the output ofthe multiplexer 1160, and may not be needed in all implementations.Additionally, the compensation signal can be generated manually by theuser guiding the placement and amplitude of the compensation signals,such as by using a mouse to place the compensation segments, and thenadjusting the amplitude with the cursor keys on a keyboard.

Multiple raster formats may advantageously be accommodated automaticallyby using multiple memory devices. Furthermore, the compensation could bemade to track changes in raster size and position by monitoring thedeflection current and high voltage.

Raster shifts resulting from changes in the ambient magnetic fields canbe corrected. Shifts in the vertical and horizontal directions resultingfrom changes in the magnetic fields along the respective horizontal axisand vertical axis of CRT 110 would result in the most noticeable changesin the compensation. The scan row and column addresses in display lists320 can be adjusted by adding or subtracting counts proportional to theintensity of the ambient magnetic fields in the respective horizontalaxis and vertical axis of CRT 110.

The invention has been explained above with reference to a preferredembodiment. Other embodiments will be apparent to those skilled in theart in light of this disclosure. For example, the present invention maybe used to effectively compensate for variations in amplitude fromcauses other than the damping wires 210 described in the preferredembodiment above. Therefore, these and other variations upon thepreferred embodiments are intended to be covered by the presentinvention, which is limited only by the appended claims.

What is claimed is:
 1. Apparatus for reducing the visibility of theshadow cast by a damping wire on the screen of a display device,comprising: a sensor device for locating said shadow on said screen ofsaid display device and responsively generate amplitude values of thearea of said screen immediately adjacent said shadow; a processor forreceiving said generated amplitude values and responsively calculatingcorrection values; and a compensator device coupled to said processorfor utilizing said correction values for reducing the visibility of saidshadow on said screen.
 2. The apparatus of claim 1 wherein acompensation engine program controls said processor.
 3. The apparatus ofclaim 1 wherein said correction values are converted into video valueswhich are alternately and sequentially stored in a plurality of displaylists along with corresponding location addresses.
 4. Apparatus forcorrecting artifacts on the screen of a display device, comprising: asensor device for locating said artifacts on said screen andresponsively generating amplitude values of the area of said screenimmediately adjacent said artifacts, wherein said artifacts may includeat least one damping wire shadow that is visible on said screen, aprocessor for receiving said generated amplitude values and responsivelycalculating correction values, said correction values being convertedinto video values which are alternately and sequentially stored in aplurality of display lists along with corresponding location addresses;and a compensator device coupled to said processor for utilizing saidcorrection values for reducing the visibility of said artifacts on saidscreen of said display device, said display device comprising a cathoderay tube.
 5. The apparatus of claim 4 wherein a shadow is located byilluminating successive scan rows on said screen of said cathode raytube and then detecting the scan row having a least amplitude value. 6.The apparatus of claim 5 wherein said correction values are calculatedby subtracting said least amplitude value from an average of two of saidamplitude values from said scan rows immediately adjacent to the scanrow affected by said damping wire shadow.
 7. The apparatus of claim 4wherein said compensator device converts said video values to analogvoltages and then applies said analog voltages to video amplifiersdriving said cathode ray tube.
 8. The apparatus of claim 1 wherein saidamplitude values are processed with a differentiation routine to locatesaid shadow on said screen of said display device, said display devicecomprising a cathode ray tube.
 9. A method for reducing the visibilityof the shadow cast by a damping wire on the screen of a display devicecomprising the steps of: using a sensor device to locate said shadow onthe screen of said display device and responsively generate amplitudevalues of the area of said screen immediately adjacent said shadow;calculating correction values from said generated amplitude values usinga processor; and utilizing said correction values to reduce the effectsof the visibility of said shadow on the screen of said display device byusing a compensator device coupled to said processor.
 10. The method ofclaim 9 wherein a compensation engine program controls said processor.11. The method of claim 9 wherein said correction values are convertedinto video values which are alternately and sequentially stored in aplurality of display lists along with corresponding location addresses.12. A method for correcting artifacts on the screen of a display devicecomprising the steps of: using a sensor device to locate said artifactson the screen of said display device and responsively generate amplitudevalues of the area of said screen immediately adjacent said artifacts,wherein said artifacts may include at least one damping wire shadow thatis visible on the screen of said display device; calculating correctionvalues from said generated amplitude values using a processor, saidcorrection values being converted into video values which arealternately and sequentially stored in a plurality of display listsalong with corresponding location addresses; and utilizing saidcorrection values to reduce the effects of the visibility of saidartifacts on the screen of said display device by using a compensatordevice coupled to said processor, said display device comprising acathode ray tube.
 13. The method of claim 12 wherein a shadow is locatedby illuminating successive scan rows on the screen of said displaydevice and then responsively detecting and locating the scan row havingthe smallest one of said amplitude values.
 14. The method of claim 13wherein said correction values are calculated by subtracting said leastamplitude value from an average of two of said amplitude values from thescan rows immediately adjacent to the scan row affected by said dampingwire shadow.
 15. The method of claim 12 wherein said compensator deviceconverts said correction values to analog voltages and then applies saidanalog voltages to video amplifiers driving said display device.
 16. Themethod of claim 9 wherein said amplitude values are processed with adifferentiation routine to locate said shadow on said screen, and saiddisplay device comprises a cathode ray tube.
 17. A computer-readablemedium comprising program instructions for reducing the visibility ofthe shadow cast by a damping wire on the screen of a display device byperforming the steps of: using a sensor device to locate said shadow onsaid screen of said display device and responsively generate amplitudevalues of the area of said screen immediately adjacent said shadow;calculating correction values from said generated amplitude values usinga processor; and utilizing said correction values to reduce thevisibility of said shadow on said screen of said display device by usinga compensator device coupled to said processor.
 18. A computer-readablemedium comprising program instructions for correcting artifacts on thescreen of a display device by performing the steps of: using a sensordevice to locate said artifacts on said screen of said display deviceand responsively generate amplitude values of the area of said screenimmediately adjacent said artifacts, wherein said artifacts may includeat least one damping wire shadow that is visible on the screen of saiddisplay device; calculating correction values from said generatedamplitude values using a processor; and utilizing said correction valuesto reduce the visibility of said artifacts on said screen of saiddisplay device by using a compensator device coupled to said processor,said display device comprising a cathode ray tube.
 19. Thecomputer-readable medium of claim 18 wherein a shadow is located byilluminating successive scan rows on the screen of said display deviceand responsively detecting and locating the scan row having the smallestone of said amplitude values.
 20. Apparatus for reducing the visibilityof the shadow caused by a damping wire on the screen of a displaydevice, comprising: means for using a sensor device to locate saidshadow on the screen of said display device and responsively generateamplitude values of the area of said screen immediately adjacent saidshadow; means for calculating correction values from said generatedamplitude values using a processor; and means for utilizing saidcorrection values to reduce the visibility of said shadow on the screenof said display device by using a compensator device coupled to saidprocessor.
 21. An apparatus for reducing the visibility of a shadow castby a damping wire on a screen of a display device, comprising: a controlmodule for providing a first address corresponding to a current locationof an electronic beam in the display device; a memory for storing adisplay list including a correction value and address corresponding to apredetermined location on the screen associated with the shadow; and acomparing module for comparing the first and second addresses, andresponsive to a match, utilizing the correction value for reducing thevisibility of the shadow on the screen.
 22. The apparatus of claim 21,wherein the correction value represents the amount of adjustment neededto compensate for the shadow.
 23. The apparatus of claim 21, wherein thedisplay list comprises first and second lists having sequences ofcorrection values, which are alternately utilized for reducing thevisibility of the shadow on the screen.
 24. The apparatus of claim 21,wherein the correction value is used to generate a correction signal,which is provided to a gain modulation input of a video amplifier forreducing the visibility of the shadow on the screen.
 25. The apparatusof claim 24, wherein the correction signal is applied to gain modulationinputs of Red, Green Blue video amplifiers for reducing the visibilityof the shadow on the screen.
 26. The apparatus of claim 24, wherein thecorrection signal is generated by manually guiding the placement ofcompensation on the screen using a cursor control device.
 27. Theapparatus of claim 21, wherein the correction value is indicative of theamount of adjustment needed to compensate for the shadow.
 28. Theapparatus of claim 21, wherein the second address is adjusted by addingor subtracting counts proportional to the intensity of the ambientmagnetic fields in the display device.
 29. The apparatus of claim 21,wherein the correction value is adjusted to track a change in rastersize and position by monitoring deflection current and voltage.